2MVA Delta - Delta Transformer

[jbs948]

I am working on a design/build project for a long-time customer of ours. They are looking for a cost effective solution (isn?t everyone) to powering some new plant equipment. Here?s what I want to do.

Install a 2MVA 4800V-480V Delta-Delta Transformer fed from an existing GE Magna-Blast breaker (I will set the overloads after the coordination study) to feed their two pieces of 480V equipment. The secondary of the transformer does not have a Center Tap, so no potential of High-Leg voltage. The 480V switchgear will consist of (1) 2500A MLO 480V 3-Phase 3-Wire Switchboard w/ 1 - 1000A 3P Fusible Switch and 1 ? 1200A 3P Fusible Switch.

Can I simply take the ?B? corner of the secondary to ground and then run the 3 phase conductors to the 480V Switchboard? Then P-P Voltages would be 480V and P-G voltages would be A-G 480V, B-G 0V and C-G 480V. Because this is a delta secondary then I wouldn?t need GFI protection either, correct?

I want to look at this as their first option because I can get a great deal on the Delta-Delta transformer and the fact I won?t have to have a GFI on the secondary. I am going to verify that the equipment can operate at this corner-grounded voltage.

I appreciate any and all feedback before I take this idea to our Principle for his blessing to move forward. I?d like to know if I?m on the right track before I go any further.

 

[charlie]

It doesn't matter which phase is connected to ground. However, the grounded conductor has to meet 200.6(B).

You may connect any part of the secondary of a 1? or 3? transformer to ground and it will work electrically. Any other connection will create a direct phase to ground fault. Unless you meet the requirements of the NEC, it will not be in compliance but it will work. For instance, ground a phase on a 208Y/120 volt transformer and it will work. 208 volts to two phases from the ground and between any two phases. From ground to neutral will be 120 volts. From any phase to neutral will be 120 volts. It works but doesn't meet the requirements of the NEC.

 

[weressl]

It doesn't matter which phase is connected to ground. However, the grounded conductor has to meet 200.6(B).

You may connect any part of the secondary of a 1? or 3? transformer to ground and it will work electrically. Any other connection will create a direct phase to ground fault. Unless you meet the requirements of the NEC, it will not be in compliance but it will work. For instance, ground a phase on a 208Y/120 volt transformer and it will work. 208 volts to two phases from the ground and between any two phases. From ground to neutral will be 120 volts. From any phase to neutral will be 120 volts. It works but doesn't meet the requirements of the NEC.

Under certain conditions you may avoid directly grounding the transformer but install a GFI monitor connecting all three phases to it so the GFI monitor will indicate in which phase your ground fault is without shutting the system down. Same requirements as the HRG of a wye system.

 

[winnie]

Do you already have the transformer and are working up a plan for its re-use, or do you have supply and load specifications and are selecting the transformer and distribution hardware?

Corner grounding is certainly acceptable, but is IMHO even less understood than 'high leg' systems.

If there are VSDs in the system, check with the manufacturer about the use of corner grounded supplies. The rectified output of a corner grounded supply 'bounces around' with respect to ground in a way that the rectified output of a grounded wye system does not.

GFP may not be _required_, but is probably worth considering. The reason for the GFP requirements is the possibility of _sustained_ arcing faults which dissipate lots of energy and do lots of damage, but which are below the OCPD pickup. In a corner grounded system, all faults are line-line faults, so the dynamics are probably different than those in wye systems, where most faults are line-neutral.

Consider high impedance grounded wye rather than corner grounded delta.

-Jon

 

[jbs948]

I have a lead on a really good deal on the transformer. It is a good deal because not many people have a need for the Delta-Delta configuration.

I have a call into the manufacturer that is supplying the equipment we're hooking up to see if it will operate on the Corner-Grounded Delta.

 

[K2500]

The secondary of the transformer does not have a Center Tap, so no potential of High-Leg voltage. The 480V switchgear will consist of (1) 2500A MLO 480V 3-Phase 3-Wire Switchboard w/ 1 - 1000A 3P Fusible Switch and 1 – 1200A 3P Fusible Switch.

Noticing the fused switches, I would add that should you go with a corner grounded system, you shoud not fuse the grounded phase.

 

[philly]

It sounds like cost is a big consideration with your project however you can add a zig-zag transformer to the secondary delta in order to derive a ground.